got meeeself a fretless about 6 months ago. i feel i'm becoming quite proficient on it. here's some things i did that were helpful....
first and most importantly, play it as much as humanly possible and don't get discouraged. it felt a little hopless for me in the beginning, but it all comes together.
- know that you body WILL LEARN the neck independent from your brain (for the most part anyhow) and if you play for hours and hours and hours it will start to come more naturally.
- as embellisher said, play to the open strings and harmonics to make sure you're playing in tune. find really simple songs that you can jam to - i just happened to have been learning Last Dance With Mary Jane when i got the bass, and it was great for practicing purposes.
- don't be afraid to slide into the notes. I thought it was lame at the beginning, but i'm finding the more I do it the quicker I can hone in on exactly what I want. slide all over the place, have fun.
- go from low to high notes on a string over and over and over till you nail them. example - play an open D and then hit an A on the G string , open D, and then a D on the G string.
- one of the coolest most fun things I did was put nylon strings on. the roundwounds my bass came with started ripping into the neck after only a couple of weeks. i have nylon flatwounds (Fender makes nylon roundwounds wich are really cool too) and it makes it a lot more fun mhwahhing around the neck.
- with the nylon strings i also found it a lot easier to do a fast vibrato kinda thing which helps me really nail the note. it's a great way to cheat as it sounds like an intentional effect i'm using and lands me right where i wanna be.
- i read in a bass player workshop to play the bass in total darkness. i did a handful of times. not sure if it did anything but i can tell ya it was hard. it must have helped me some. felt meditative.
i love my fretlesses - i wound up buying another.
last note - if it's a lined neck, your battle is half over. there is a HUGE difference IMO between playing a lined and unlined fretless. the first one I got was unlined, i still have fun wrestiling with it. the lined i was able to play as soon as i got it, i guess from all the practice i had gotten on the unlined. I love them both.